I added a small algorithm for parsing an+b values at
<http://dev.w3.org/csswg/css3-syntax/#parse-anb-notation>. It's just
a "turn everything back into a string and reparse" algorithm.
Does it look good? Alternately, I could do it by looking at the
original tokens, it's just a bit messier that way. There's 6
different ways a valid an+b can be tokenized, and one of them involves
a dimension token with a unit matching /n[+-]\d+/.
~TJ